Presentation on theme: "5116/05 SCIENCE PRACTICAL EXAM 18 th October 2012 (Thursday)"— Presentation transcript:
5116/05 SCIENCE PRACTICAL EXAM 18 th October 2012 (Thursday)
What are you expected to do? You are usually asked to do the following: - heat a substance - add a substance to another - describe a sample
What must you observe? You must be alert to observe the following: - changes in colour - production of gases - formation of precipitate
NOTES FOR QUALITATIVE ANALYSIS
Test for Gases ObservationsConclusion
Test for Gases – Ammonia (NH 3 ) Observations: Effervescence observed. Gas evolved turns damp litmus paper blue. Conclusion: Ammonia gas is evolved.
Test for Gases – Carbon Dioxide (CO 2 ) Observations: Effervescence observed. Gas evolved gives white ppt. with limewater. Conclusion: Carbon dioxide gas is evolved.
Test for Gases – Chlorine (Cl 2 ) Observations: Effervescence observed. Gas evolved turns damp litmus paper red, then white. Conclusion: Chlorine gas is evolved.
Test for Gases – Hydrogen (H 2 ) Observations: Effervescence observed. Gas evolved extinguishes a lighted splinter with a ‘pop’ sound. Conclusion: Hydrogen gas is evolved.
Test for Gases – Oxygen (O 2 ) Observations: Effervescence observed. Gas evolved relights a glowing splinter. Conclusion: Oxygen gas is evolved.
Test for Gases – Sulfur Dioxide (SO 2 ) Observations: Effervescence observed. Gas evolved turns aqueous acidified potassium dichromate (VI) from orange to green. Conclusion: Sulfur dioxide gas is evolved.
Test for Cations ObservationsConclusion
Test for Cations – Na + /K + /NH 4 + Observation: No ppt. observed. Observation: No visible reaction on warming. Conclusion: Na + / K + is present. Observation: Ammonia produced on warming. Conclusion: NH 4 + is present.
Test for Cations – Ca 2+ / Pb 2+ / Zn 2+ Observation: White ppt. observed. Observation: White ppt., soluble in excess, giving a colourless solution. Conclusion: Pb 2+ / Zn 2+ is present. Observation: White ppt., insoluble in excess. Conclusion: Ca 2+ is present.
Test for Cations – Cu 2+ / Fe 2+ / Fe 3+ Observation: Light blue ppt., insoluble in excess. Conclusion: Cu 2+ is present. Observation: Red-brown ppt., insoluble in excess. Conclusion: Fe 3+ is present. Observation: Green ppt., insoluble in excess. Conclusion: Fe 2+ is present.
Test for Anions Observations Conclusion
Test for Anions – Cl - Test: To 2 cm 3 of solution, add an equal amount of dilute nitric acid, followed by aqueous silver nitrate. Observation: White ppt. observed. Conclusion: Cl - is present.
Test for Anions – SO 4 2- Test: To 2 cm 3 of solution, add an equal amount of dilute nitric acid (dilute hydrochloric acid), followed by aqueous barium nitrate (aqueous barium chloride). Observation: White ppt. observed. Conclusion: SO 4 2- is present.
Common Oxidising Agents OXIDISING AGENTBEFOREAFTER Aqueous acidified potassium manganate (VII) H + / KMnO 4 (aq) Purple (MnO 4 - ) Colourless (Mn 2+ ) Aqueous acidified potassium dichromate (VI) H + / K 2 Cr 2 O 7 (aq) Orange (Cr 2 O 7 2- ) Green (Cr 3+ ) Aqueous iron (III) compoundsYellow (Fe 3+ ) Pale green (Fe 2+ ) Aqueous iodineDark brown (I 2 ) Colourless (I - ) Oxidising agents undergo reduction.
Common Reducing Agents REDUCING AGENTBEFOREAFTER Aqueous iron (II) compoundsPale green (Fe 2+ ) Yellow (Fe 3+ ) Aqueous potassium iodideColourless (I - ) Dark brown (I 2 ) Reducing agents undergo oxidation.
A more reactive metal displaces a less reactive metal from its solution. DISPLACEMENT REACTIONS
Common Metals MetalColour of MetalColour of Metallic Salt Solution MagnesiumShinyColourless (Mg 2+ ) AluminiumShinyColourless (Al 3+ ) ZincDull GreyColourless (Zn 2+ ) IronDark greyPale green (Fe 2+ ) / Yellow (Fe 3+ ) LeadDull greyColourless (Pb 2+ ) CopperReddish-brownBlue (Cu 2+ ) SilverShinyColourless (Ag + )
Common Metals magnesium ribbon aluminium foil zinc powder lead iron filings copper turnings
Displacement Reactions Magnesium Mg (s) + CuSO 4 (aq) → MgSO 4 + Cu (s) Effervescence observed. The magnesium ribbon dissolves rapidly into the solution. The blue solution turns colourless slowly. A reddish brown solid is deposited at the bottom of the test-tube.
Displacement Reactions Magnesium Mg (s) + FeCl 2 (aq) → MgCl 2 + Fe (s) Effervescence observed. The magnesium ribbon dissolves rapidly into the solution. The pale green solution turns colourless slowly. A grey solid is deposited at the bottom of the test- tube.
Displacement Reactions Zinc Zn (s) + CuSO 4 (aq) → ZnSO 4 + Cu (s) Effervescence observed. The zinc powder dissolves slowly into the solution. The blue solution turns colourless. A reddish brown solid is deposited at the bottom of the test-tube.
Displacement Reactions Zinc 3 Zn (s) + 2 FeCl 3 (aq) → 3 ZnCl Fe (s) Effervescence observed. The zinc powder dissolves slowly into the solution. The yellow solution turns colourless. A grey solid is deposited at the bottom of the test- tube.
Displacement Reactions Copper Cu (s) + 2 AgNO 3 (aq) → Cu(NO 3 ) 2 (aq) + 2 Ag (s) The colourless solution turns blue. A silvery, shiny solid is deposited at the bottom of the test-tube.
Recording Observations The examination format usually comes in the form of a table, with answer spaces opposite the instructions for each test or experiment. Your observations must be recorded directly opposite these instructions, so that it is clear which test your observations apply to. This is important when the test instructions consist of two or more separate steps.
Recording Observations Observations are changes which are seen in tests. These changes are: Colour changes in solids or liquids, Precipitates formed, Effervescence (bubbles) of gases evolved, and if they are coloured or have strong smells, Flames produced.
Recording Observations A gas is being produced if you can see bubbles in a liquid or if you can observe its colour or detect its strong smell. When a gas is observed, it must be identified with a test. You must then record the name of the gas, its colour and smell (if any) and the test which identifies it. When testing for a gas, make sure the gas is still being produced. There is no point in testing for a gas after it has all disappeared. Gases produced by reagents alone are not observations.
Recording Observations When carrying out an experiment, always leave the test-tube to stand for one or two minutes as some changes take a slightly longer time. There is an observation for every test. If you do not observe any change, then you should record ‘no visible change’. Do not leave a blank space in your record.
Recording Conclusions You are usually expected to draw conclusions from the results of your tests. These conclusions consist of a number of deductions from your observations concerning: The identity of the substances in the unknown, such as cations and anions present, The nature of the unknown, such as whether or not it is an oxidising agent or a reducing agent, The elements present in the unknown.
Recording Conclusions It is important to realise that it may not be possible to completely identify the unknown from the result of your tests, because: either you do not have enough facts to work it out, or the unknown is a substance not in the O-level syllabus.
General Comments from Examiners Candidates should be encouraged to observe carefully and record all changes, not just the first ones they see. Generally, a question which instructs candidates to continue until no further change is seen indicates that more than one observation can be made. Candidates should be reminded to use the correct terminology; for example, the words ‘precipitate’, ‘solution’ and ‘mixture’ have specific meanings in chemistry.
General Comments from Examiners When conducting a flame test, candidates should focus on the colour of the flame, and record accordingly. Some candidates wrongly reported that the flame burnt, or relit, or that the solid turned black.
General Comments from Examiners Ambiguous reporting You must always report on the colour and state (whether it is a precipitate or a solution) when describing a result). Incorrect ( )Correct ( ) It turns blue.A blue ppt. is formed. The xxx solution turns blue. A solid is formed.A white ppt. is formed. The filtrate is clear.The filtrate is a colourless solution. The solution is clear.The white solid dissolves completely to give a colourless solution.
General Comments from Examiners Incomplete reporting You must always report completely to let the examiners know how you arrive at a conclusion. Incorrect ( )Correct ( ) Carbon dioxide gas is evolved. Effervescence observed. Gas evolved gives a white ppt. with limewater. The precipitate dissolves.Light-blue ppt., soluble in excess, giving a dark blue solution.
General Comments from Examiners Incorrect reporting The common errors involve the use of the term white / clear to mean colourless. Incorrect ( )Correct ( ) The solution becomes white.The white solid dissolves completely to give a colourless solution.